There are basically two types of practices predominantly utilized for the retreading of worn vehicle tires. One practice involves applying uncured rubber to a worn tire casing and then bonding such rubber to the tire casing and simultaneously forming a tread pattern in that rubber by heating and vulcanizing the rubber in a pattern forming mold. The other practice involves applying to the worn tire casing a strip of precured and premolded tread rubber in which a tread pattern has been previously formed and bonding that strip to the worn tire casing by use of heat and pressure. In this second practice, the curing and molding of the tread rubber prior to its bonding to the tire casing permits such curing and molding to be done at much greater pressures and temperatures than are usually possible when the curing and molding of the tread rubber are accomplished with the tread rubber on the tire casing, as are done in the first practice of retreading described above. Accordingly, this second practice of retreading yields a retreaded tire in which the tread rubber is generally denser and more durable.
To further explain how this practice of retreading is carried out, unvulcanized rubber, commonly referred to as cushion gum, is typically used as a bonding material to bond the precured and premolded tread rubber to the tire casing. This bonding material may be supplied already adhered to the under surface of a continuous strip of precured tread rubber or it may be supplied separately in strip form itself for application to the tire casing in preparation for application of the strip of precured tread rubber. Whichever is done, these materials are then subjected to heat and pressure to vulcanize the bonding material and permanently bond the tread rubber to the tire casing.
One favored manner for applying this heat and pressure involves placing the tire casing, onto which the bonding material and tread rubber have been applied, into a flexible, air impervious rubber envelope, and then placing this entire tire assembly into a curing chamber for bonding. The rubber envelope is utilized to aid in holding the bonding material and tread rubber in intimate contact with the underlying tire casing during curing and to also aid in properly applying bonding pressure to the tread rubber. However, upon the tire assembly being placed in the envelope, there is generally always some air entrapped between the surface of the tread rubber and the overlying envelope, so that when the enveloped tire assembly is placed in the curing chamber and heated to effect curing of the bonding material, this entrapped air expands and forms air pockets which arise under the envelope at various locations. There are thus formed localized points of increased pressure on the interior of the envelope which are also localized points of increased temperature. As a result, the envelope tends to prematurely rupture at such points, and as a further result the tire assembly then being processed is not properly cured and bonded so that it usually must be then stripped down to its components and rebuilt, all resulting in substantial loss of time, labor and material.
Notwithstanding these undesired results, it does not appear that their true causes have been fully appreciated before. Clearly, these undesired results have not been satisfactorily avoided. To the extent that there have been some efforts made to attempt to eliminate a portion of the air entrapped within the envelope, these efforts have been primarily limited to removing the air entrapped between the tire casing and tread rubber in the area of the bonding material to try to improve the bonding of the tread rubber to the tire casing. One such attempt has involved the placement of a network of spaced apart porous cords across the crown of the tire casing, and thereafter covering the cords with the bonding material and tread rubber. Another attempt has involved the placement of porous fabric straps adjacent the side walls of the tire casing, but not extending across the tread strip. Neither of these attempts have accomplised any removal of air from the face of the tread strip since the materials used therein have not contacted this area.
Other apparent attempts, while still having as their primary goal the removal of air from the bonding material area, have provided some minimal contact with the face of the tread strip to accomplish some minimal wicking of air therefrom. One of these attempts has involved extending the fabric straps mentioned above across and over the top of the tread strip. Another, described in U.S. Pat. No. 3,779,833 to Reppel, has involved fitting a nylon mesh bag around the entire prepared tire prior to applying the air impervious rubber envelope around the tire and this nylon bag. Yet, again, such attempts have had as their primary goal the elimination of air from the bonding area, as opposed to the tread strip, which has been only minimally contacted.
Finally, one other apparent attempt has involved a network of widely spaced apart porous cords applied across the tread strip. But even this attempt has involved only minimal contact with but a small portion of the face of the tread strip thereby resulting in minimal air wicking from that area and leaving the basic problems of premature envelope failure and unduly restricted curing temperatures still existing.